Conventional satellite tracking has been performed through control of a drive axis to direct a beam of the antenna toward a satellite using a pre-obtained forecast value of satellite orbit. As a technique for controlling the attitude of an antenna (ground antenna) of a satellite communication earth station, an azimuth-elevation (AZ/EL) mount may be used for supporting rotation around a vertical axis (AZ axis) and a horizontal axis (EL axis).
In the case of an AZ/EL mount, sometimes it is necessary to rotate the mount instantly around the AZ axis by 180° when the target object passes the zenith. This is commonly referred to as gimbal lock. Patent Literature 1 describes an antenna apparatus that does not experience gimbal lock. The antenna apparatus in Patent Literature 1 performs switching so that when a beam direction of an antenna is less than or equal to a set elevation angle, inputs are given to drive inputs of two axes among a vertical axis, a horizontal axis, and a cross horizontal axis that are individually driven, whereas when the beam direction of the antenna is greater than or equal to the set elevation angle, inputs are given to drive inputs of all of the three axes. Also, after the switching to the three-axis driving, a value for each specific axis obtained by calculating the present values of the three axes is provided to the drive input of each specific axis of the three axes.
Also, when AZ/EL coordinates are used in an interpolation process while the satellite is in a position on an orbit near the zenith, compared to when the satellite is in any position on an orbit besides near the zenith, the rate of change in an angle of each axis per Δt sec, which is the calculation cycle for controlling the antenna, greatly increases and the error of an obtained program command angle greatly increases. In order to avoid this, a technique for satellite tracking antenna drive control in Patent Literature 2 discloses a satellite tracking antenna drive control technique in which a program prediction angle for pointing to the satellite is calculated at predetermined intervals based on orbit prediction information of the satellite. Then, when the satellite is predicted to pass near the zenith based on the program prediction angle, X/Y coordinates are chosen as coordinates used in an interpolation process to calculate a program command angle pointing to the satellite at a time interval shorter than the time interval for this program prediction angle.